A maximum accuracy of determining the coordinates of objects makes it necessary that the Doppler frequency should be continuously measured over the entire period of time the satellite is within the visibility zone of the object.
The Doppler frequency is measured by counting the number of Doppler signal periods contained within a known period of time referred to as the sampling or measurement interval. No Doppler signal is formed during a brief loss of signal from the navigation satellite. The count during this period is wrong and cannot be used to calculate the Doppler frequency. During the processing stage, each count gives a position line on the Earth's surface on which the object is found. The greater the number of such position lines, the higher the accuracy with which the coordinates of the object are determined. The loss of each measurement interval affects the object localization accuracy; the loss of a substantial number of such intervals makes the solution of a navigation problem an impossibility.
Today, extensive use is made of satellite navigation systems of the type that comprises data processing means and a receiver of signals sent by a satellite and carrying information on the orbital position of the satellite and orbit parameters. The measuring parameter in such systems is the Doppler shift of the carrier frequency of the transmitter borne by the satellite.
There is known a double-channel satellite navigation system (cf. the JMR publication "The JMR-1 Doppler Survey Set. Description and Application", 1976) comprising a mixer whose imputs receive a signal arriving from a navigation satellite and a signal arriving from an automatic phase frequency control unit. The output of the mixer is electrically coupled via an amplifier to a signal level analyzer, a data selection unit, a Doppler frequency measuring unit and an automatic phase frequency control unit. Reference-frequency voltage for the automatic phase frequency control unit is supplied by a reference-frequency oscillator. From the outputs of the signal level analyzer, data selection unit and Doppler frequency measuring unit, information is supplied to respective inputs of a data recorder.
In case of a drop, however brief, of the level of the satellite's signal below a point which insures normal operation of the system, the Doppler count during the entire measurement interval is wrong and cannot be used to solve the navigation problem. The result is a poor accuracy of determining the coordinates of the object.
There is further known a double-channel satellite navigation system wherein each channel comprises a mixer. The inputs of the two mixers receive the high and low frequency components, respectively, of a double-frequency signal sent by a navigation satellite. Each of the two mixers has its output electrically coupled to a first input of a phase detector whose second input is connected to a reference-frequency oscillator. The output of the phase detector is electrically connected to an input of a voltage-controlled oscillator whose output is connected to an input of a frequency multiplier. The latter has its output connected to a second input of the mixer and to an input of a Doppler frequency measuring unit. A signal from the output of the Doppler frequency measuring unit is sent to a computer. The mixer has its output electrically connected to an input of a signal level analyzer and to an input of an ephemeris information selection unit. Signals from the outputs of the latter two units are transmitted to a computer (cf. the Magnavox publication: "Technical Manual. Operation and Service Instructions for Satellite Navigation Set MX 702A", 1975).
In the system under review, the output of the phase detector is connected to the input of the voltage-controlled oscillator via a low frequency filter.
In the case of a drop, however brief, of the level of the satellite's signal below a point which insures normal operation of the system, the Doppler count during the entire measurement interval is wrong and cannot be used to solve the navigation problem. As in the case of the previously discussed system, this affects the accuracy of determining the coordinates of objects.